Global challenges Study Guide

Crude oil is a mixture of many different hydrocarbons, which are compounds made only of hydrogen and carbon atoms. These hydrocarbons have different chain lengths and structures, leading to varying boiling points. Fractional distillation is a process used to separate crude oil into useful fractions based on these differences in boiling points. The crude oil is first heated in a furnace until most of it vaporises. The hot mixture of liquid and vapour is then pumped into the bottom of a tall fractionating column, which is hotter at the bottom and cooler at the top. As the hot vapours rise up the column, they cool. Hydrocarbons with higher boiling points condense at lower levels in the column where it is hotter, while those with lower boiling points continue to rise higher before condensing at cooler levels. Non-vaporised crude oil, containing the heaviest fractions, collects at the bottom. The different fractions, such as bitumen, fuel oil, diesel, kerosene, gasoline (petrol), naphtha, and refinery gases, are collected at various levels as liquids. This process allows for the separation of crude oil into products with specific uses, ranging from road surfacing to fuels and chemical feedstocks.

Cracking is a vital industrial process used to break down large, long-chain hydrocarbon molecules, found in crude oil, into smaller, more useful hydrocarbon molecules. Crude oil is a mixture of hydrocarbons of different lengths. While some long-chain hydrocarbons, like those found in fuel oil or bitumen, are useful, there's a much higher demand for shorter-chain hydrocarbons such as petrol (gasoline), naphtha (used for making plastics), and gases like ethene and propene. These smaller molecules are in high demand because they are more valuable fuels and crucial raw materials for the petrochemical industry.

The cracking process typically involves two main methods: thermal cracking and catalytic cracking. In thermal cracking, high temperatures (around 400-900 degrees Celsius) and high pressures (up to 70 atmospheres) are used to break the strong C-C covalent bonds in the larger molecules. This process often produces a high proportion of alkenes, which are highly reactive and essential for making polymers. Catalytic cracking, on the other hand, uses a lower temperature (around 450-550 degrees Celsius) and lower pressure, relying on a catalyst (often zeolites, which are aluminium silicates) to speed up the reaction and allow it to occur more efficiently at less extreme conditions. Catalytic cracking primarily produces branched alkanes and cycloalkanes, which are ideal components for high-octane petrol. Both methods achieve the same goal: converting less valuable, long-chain hydrocarbons into more valuable, shorter-chain hydrocarbons that meet market demand for fuels and chemical feedstocks. This transformation prevents wasted resources and maximises the value derived from crude oil.